1. Field of the Invention
This invention relates generally to inoculants which are useful for introducing low temperature boiling metals into a molten iron or molten steel bath, and more particularly to an inoculant which is a lamellar product of a mixture of two or more metals with graphite.
2. Description of the Prior Art
Low boiling point metals are often needed for various purposes in cast iron production or in steel production. Different low boiling point metals, of course, provide different functions in the iron and steel production. For instance, in steel production, calcium is usually added to the molten bath as a deoxidizer or desulfurizer. In the production of cast iron, a magnesium, calcium or a rare earth metal must be introduced into the molten bath of iron carbide to graphitize the carbide or to effect "nodularization" of the cast iron, which imparts the desirable ductility characteristic of good quality cast iron.
These additives are introduced into the molten bath and volatilized. As the vapors move upward toward the surface of the bath, they enter into reactions with various components of the bath. Unreacted additives escape into the atmosphere and are recovered. It is known, of course, to try to control the rate of volatilization of the additives to provide a more efficient utilization of additives and with consequent reduction in losses.
One technique for effecting such control in the prior art, has been to introduce the volatile metals beneath the surface of the bath by means of a lance, a closed ladle or the like. This method however, is largely ineffective since the metals vaporize so quickly that they soon rise to the surface and dissipate into the atmosphere. This is especially a problem in cast iron production, since if the metal additive volatilizes out of the bath too quickly, the carbide of the batch will be insufficiently graphitized, and the resulting product will not be homogeneous.
To avoid this difficulty, it has been suggested to inoculate molten iron with an alloy which has a higher boiling point than the metal alone. The problem with this technique however, has been the expense of making such alloys, and the risk that the alloying metal will contaminate the product iron or steel. Snow, U.S. Pat. No. 3,321,304, describes still another technique for attempting to control the volatilization of the additive metals. According to the Snow patent, pores of a porous refractory, such as porous coke, graphite, carbon, silicon carbide or the like, is impregnated with a low boiling point metal additive. The difficulty with the Snow technique however, is that volatilization of the metal within the pores will be almost as rapid as volatilization of the free metal itself.
The technique of Snow differs from that of the present invention in that the Snow technique does not lead to metals homogeneously dispersed in the graphite. Thus, the volatilization of any metal which enters the interior portion will be insignificant as compared with the volatilization of the free metal from the pores.
The difficulty with all of these prior art techniques has been that none of them provide for an effectively predictable and controlled quantity of volatilized metal to be introduced into the molten bath, which can be relatively standardized from batch to batch. While the prior art use of free metal alone will provide a predictable quantity of volatilized metal into the bath, the rate of volatilization of free metal is explosively rapid and most of the metal will be lost to the atmosphere. Moreover, even with the addition of a known quantity of free metal to the molten bath, it is completely a matter of chance as to how much of the volatilized metal will remain available for reaction in the molten bath, and how much of the volatilized metal will escape into the atmosphere.
In prior art attempts at control such as disclosed in the Snow patent, while it is less a matter of chance as to how much of the volatilized metal is available for reaction in the molten bath, it is still largely unpredictable from batch to batch as to how much of the volatilized metal will remain in the molten bath for reaction and how much will escape into the atmosphere. The reason for the inability to predict in the Snow procedure is that the size of the pores of the porous refractory can only be estimated and will obviously vary from one porous refractory specimen to another. Thus, the quantity of free metal loaded into the refractory will greatly vary. Moreover, the rate of release will depend on multiple factors pertaining to the nature of the refractory, which of course cannot be standardized.
A need therefore continues to exist for a technique of delivering a low boiling point, highly volatile metal additive to the molten bath of iron or steel, wherein a more predictable, controlled release of volatilized metal can be added to the batch, which will yield a more uniform result from batch to batch of molten metal.